The present invention relates to an image signal coding apparatus used in a facsimile system, and image electronic file, or the like.
In an image transmission system (e.g., a conventional facsimile system) or an image file system using a recent optical or magnetic disk, an image signal is coded to decrease the number of data signals, and data transmission and data storage (i.e., packetting) can be effectively performed at high speed.
For example, in a facsimile, MH (Modified Huffman) coding is known as a one-dimensional coding scheme; MR (Modified READ) coding, a two-dimensional coding scheme; and MMR (Modified Modified READ) coding, an efficient two-dimensional coding scheme.
The MH, MR, and MMR schemes have the following relationship. The MMR scheme includes techniques similar to those of the MH scheme, and is obtained by partially modifying the MR scheme.
Images to be coded and coding rules comply with T4 and T6 recommended by the CCITT (International Consultative Committee for Telephone and Telegraph).
The MMR scheme is described as an efficient two-dimensional coding scheme in a recommended communications system (Posts and Telecommunications No. 197) for Facsimile Group No. 4 Devices on page 52 and the subsequent pages, in the Official Gazette (Extra No. 29) dated Mar. 22, 1985.The MH scheme is described as a one-dimensional coding scheme and the MR scheme as a two-dimensional coding scheme both in Notification No. 1013 of the Ministry of Posts and Telecommunications, 1981.
Among the coding schemes described above, the MR and MMR schemes utilize a correlation between an image signal of a given line and an image signal of the immediately preceding line to code the image signal of the given line. Correlation discrimination requires complex processing. This processing is performed by software processing using a microcomputer or the like. Software processing requiring several steps to discriminate the correlation has an upper limit to its coding speed. An original image is photoelectrically read to produce an image signal. The image signal is temporarily stored in a large-capacity memory. The image signal is then read out and coded in synchronism with a coding operation. It is therefore difficult to code an image signal output such as a read image signal without delay, i.e., on a real-time basis.
In addition, since the coding operation is performed in a code corresponding to a correlation discrimination result, it is difficult to obtain an image signal by photoelectrically reading an original image without delay.
In order to represent a relatively long run length in a horizontal mode according to the MMR scheme, a plurality of coding codes are required. If these coding codes are generated after mode discrimination, the coding operation speed is lower than the image signal input speed. Therefore, it is difficult to perform a real-time coding operation.
When the coding operation is performed after correlation discrimination, pixels preceding and succeeding an input pixel must be monitored. In order to synchronize the coding operation with the image signal input, coding must be performed as soon as the correlation discrimination is completed.
In image signal coding using the MH, MR, and MMR schemes, the number of succeeding pixels of image signals representing an identical color, that is, a run length, must be represented by a code. More specifically, a run length is represented by a terminating code representing one of run lengths of 1 to 63, and a make-up code representing one of run lengths of 64, 128, 192, . . . , 64x M, . . . , 2560. Therefore, in order to express a relatively long run length, a plurality of codes are required. However, if these codes are simultaneously generated, a coding circuit is overloaded and requires a long processing time.
The MH, MR, and MMR schemes are selectively used according to the type of subsequent processor used for processing the coded image signal. If the subsequent processing format is changed, a circuit modification is required to be compatible with the format change, thus degrading coding efficiency. Since the three coding schemes are related to each other, as described above, demand has arisen for using these three coding schemes in a single coding apparatus.